We have found that if core regions crucial for class II binding are incorporated in multiple copies in the same peptide molecule ("reiterative motifs"), marked enhancement of the binding capacity occurs. Isotype specificity (IAd vs IEd binding capacities) is retained in all three antigenic determinants so far analyzed (lambda rep 12-26, OVA 323-339, and hen egg lysozyme 105-120). The mechanism involved in such an effect is not clear, but experiments involving introduction of a peptide spacer between two repeated core regions do not support the notion that the effect is mediated by cross-linking of more than one MHC molecule, favoring the possibility that conformational effects or distinct subsites of interaction on the MHC molecule may be involved. Based on reiterative structures, a peptide molecule composed of only two different amino acids (Ala and His) has been produced that still retains a very high binding affinity. An 125I-radiolabeled form of this peptide has been used to demonstrate that the high binding detected is mediated by the same binding site involved in the interaction of IAd and OVA 323-339. Inhibition of Ag presentation studies further supports the immunologic relevance of the phenomena observed. Finally, we observed naturally occurring clustered binding sites in proximity of immunodominant protein regions, raising the possibility that the phenomenon might have a physiologic counterpart.